A response to Leon Ashby's "Why an ETS is not necessary": Slide 19

Slide 19: What is
Earth’s normal temperature?

Over the
4.6 billion years of its existence, the Earth’s temperature has varied
widely, between Snowball
Earth conditions when the land and oceans may have been entirely frozen
over, to Greenhouse Earth episodes when forests grew at the
poles.

Since
1950 the average temperature has already risen
0.6°, probably already exceeding the highest experienced during
civilisation, and it is still rising fast each
decade. Without urgent and strong global action, global average temperature
is projected to rise between
2°C and 6°C by 2100.

Mediaeval warm period

The
IPCC’s 1990 report is not available online, but the 2007 report concludes:

“[M]edieval warmth was heterogeneous in terms of its precise timing and
regional expression. ...the evidence is not sufficient to support a conclusion
that hemispheric mean temperatures were as warm, or the extent of warm regions
as expansive, as those in the 20th century as a whole, during any period in
medieval
times.

A
recent scientific
paper summarised in the Skeptical
Science blog supports the IPCC’s conclusion that the Mediaeval Warm
Period was regional, not global. It involved warmer conditions over part of the
North Atlantic, Southern Greenland, the Eurasian Arctic and parts of North
America. Other regions, such as central Eurasia, north-western North America and
the tropical Pacific were substantially cooler.

Interestingly
the figure Ashby uses in slide 20 below
shows a Mediaeval Warm Period running from 800-1000 AD, rather than 1100-1300 AD
as shown here. This also supports the IPCC’s conclusion that the Mediaeval
warming was not a global phenomenon, but occurred at different times in
different places.